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RPM vs Speed

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    Posted: September-30-2017 at 6:03pm
Got the boat out for the first time this morning since most of the new stuff ( 95 SN, GT40) - prop shaft, prop, gauges, etc. All but the speed could be checked at home of course, but I felt I needed to check the gauges on the lake, specifically the GPS speedometer to make sure all was good. GPS checks out perfect, but I was curious about the speed vs the RPM as my tach registered about 2500 @ 32mph. Didn't go WOT yet as they said the engine only had abut 3-4 hours on it when I bought it, but I did push it up to about 3500 - 3600 and was seeing around 42. This seemed a bit fast for 3500, so I was then wondering about the tach, if I had it set properly. Thoughts?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 8122pbrainard Quote  Post ReplyReply Direct Link To This Post Posted: September-30-2017 at 6:54pm
Joe,
Get a shop tach on it and see how it compares.


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Post Options Post Options   Thanks (0) Thanks(0)   Quote KENO Quote  Post ReplyReply Direct Link To This Post Posted: September-30-2017 at 7:06pm
If you Google "correctcraftfan 95 ski nautique speed vs rpm" you'll keep yourself busy reading for quite a while.

Or you can leave off the correctcraftfan part and get numbers from that "other" website and from ball of spray also.

You'll even find your old post titled "correct prop size"

Most all of the info would suggest roughly a 100 to 1 relationship meaning 3600 rpm would be about 36 mph
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Post Options Post Options   Thanks (0) Thanks(0)   Quote KENO Quote  Post ReplyReply Direct Link To This Post Posted: September-30-2017 at 8:42pm
A couple of other things

You mentioned tach settings, I don't know what you have, maybe a 4-6-8 selector but it must be set for 8 right now otherwise if it was on 4 or 6 it would be reading higher than actual RPM which shouldn't pertain to your situation

You have either a low reading tach or a high reading GPS or some combination of both.

I don't think you ever said what you had for a prop now in any of your posts, I just see a picture of an ACME but no numbers in an old post, but I figure there's probably nothing that would give numbers like you're seeing for speed vs rpm.

So..............what's the new prop?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Orlando76 Quote  Post ReplyReply Direct Link To This Post Posted: September-30-2017 at 8:53pm
Just curious as to what gps speedo you have?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote KENO Quote  Post ReplyReply Direct Link To This Post Posted: September-30-2017 at 9:19pm
Here's a Faria troubleshooting guide

link

Maybe page 12 will come in handy.............or maybe it won't.

I have a phone GPS that I use when I want to feel "speedy". It reads 5 mph high. Hope your phone GPS that you're comparing the installed one to is better than mine
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Post Options Post Options   Thanks (0) Thanks(0)   Quote gt40KS Quote  Post ReplyReply Direct Link To This Post Posted: September-30-2017 at 9:37pm
Sorry Ken, I thought I had posted the prop previously. It's a 422 that I picked up from a buddy, virtually new with only about a half hour test time and looks pristine.   As for the GPS speedo, I confirmed it's accuracy with an app on my phone, which was in turn confirmed using the GPS nav system in my truck. I should have snapped pics of the pair while driving, didn't think of that at the time. It's possible I've got the wrong #'s I suppose, I'll check it again next time out. The new tach though has a lot more settings that I remember from the old days and they don't necessarily correspond to the # of cylinders. One even spec'd "mercruiser v8 only"

I'll have to pick up a reliable meter or shop tach to check it further. I have a cheap tach meter that I used to set the idle on a set of SeaDoo's but I doubt it will be accurate enough for this.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote gt40KS Quote  Post ReplyReply Direct Link To This Post Posted: September-30-2017 at 9:47pm
Originally posted by Orlando76 Orlando76 wrote:

Just curious as to what gps speedo you have?


Faria 4"

JCCI
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Post Options Post Options   Thanks (0) Thanks(0)   Quote baitkiller Quote  Post ReplyReply Direct Link To This Post Posted: September-30-2017 at 9:50pm
Pete is right. Listen to the Professor $11 at Amazon. i use one every day and its dead on balls accurate.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Gary S Quote  Post ReplyReply Direct Link To This Post Posted: September-30-2017 at 11:17pm
Originally posted by KENO KENO wrote:

Here's a Faria troubleshooting guide

link

Maybe page 12 will come in handy.............or maybe it won't.


Last paragraph on page 11 is helpful too......
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Post Options Post Options   Thanks (0) Thanks(0)   Quote KENO Quote  Post ReplyReply Direct Link To This Post Posted: October-01-2017 at 12:32am
when you get right down to it, page 12 sends ya' back to page 11 in the reading high and reading low sections.

Like Gary said that paragraph is real helpful

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Post Options Post Options   Thanks (0) Thanks(0)   Quote gt40KS Quote  Post ReplyReply Direct Link To This Post Posted: October-01-2017 at 2:47am
Well, go figure - didn't read the part about the gauge having a fine tune adjustment.
However, for the most part the mystery is solved. It seems I had the setting wrong anyway.    I found one of my old Bosch competition tachs and set it up as the "shop" device for a first test. As Ken figured, there was quite a marked difference - 1550 on the Bosch, 1200+ on the Faria. For whatever reason, this gauge had 2 different setting labels on it ... mistake? I don't know, but I had set it according to the label on the side of the gauge just under the adjustment dial which said 8 cyl - position 4. It's also the one that had the special Mercruiser setting. The label on the back has 8 cyl as position 3 and 10 cyl on position 4.   So reset to #3 and tested again - Bosch, 1500+ and Faria 1450. Still a bit low, but now I know there's a fine adjustment ( thanks Gary ) so I'll be doing that tomorrow.




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Post Options Post Options   Thanks (0) Thanks(0)   Quote KENO Quote  Post ReplyReply Direct Link To This Post Posted: October-01-2017 at 10:00am
I think that position 3 is right which leads to the question of what's more accurate, your old Bosch or your new Faria?

I'm not too sure I'd adjust the Faria to match the Bosch

I am sure I'd head out on the lake and get some numbers before thinking about doing any adjusting, but that's just me
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Post Options Post Options   Thanks (0) Thanks(0)   Quote gt40KS Quote  Post ReplyReply Direct Link To This Post Posted: October-01-2017 at 3:24pm
Perhaps you're right Ken. I think I'll get out that cheap digital induction meter I used for the skis today .... If the $11 Amazon model is good enough for Baitkiller to use every day I guess the EZtach should be close enough to see if I even need to worry about it.
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